It is an extraordinary accomplishment that most developing neuronal networks achieve an appropriate level of excitability, during a dynamic period of embryonic development when there are several challenges to a network's excitability (e.g. cells increase in size). Therefore it is not surprising that the incidence of seizure activity is higher in the neonatal period than in any other age group. Neonatal seizures are often the first indication of neurological dysfunction, and are strong predictors of long-term cognitive and developmental impairment. Understanding the rules and mechanisms that underlie the maturation of network excitability are therefore essential. In the last decade an exciting new field has emerged that provides critical insights to understanding the rules that networks follow in order to achieve appropriate levels of activity. Many studies have now shown that networks homeostatically maintain activity levels within an appropriate range by adjusting synaptic strength (homeostatic synaptic plasticity). The vast majority of these studies have blocked network activity in vitro (culture) for days, and although activity doesn't homeostatically recover, changes in synaptic strength are in a compensatory direction. Therefore, it is assumed, although untested, that these changes would normally act to recover activity levels following perturbations in vivo. Compensatory changes in intrinsic cellular excitability (cell's responsiveness to synaptic input) also likely contribute to the homeostatic process, although these changes have received far less attention than synaptic compensations. We have found that changes in cellular excitability are faster and likely more important in the initial recovery of normal activity levels. The objective of this application is to better understand the role of cellular excitability in the process of homeostatically recovering and maintaining network activity in developing networks. We are proposing to perturb network activity in the living embryo, allow for the homeostatic recovery of activity and assess how changes in cellular excitability contribute to this recovery. This will provide a more realistic, comprehensive understanding of the homeostatic regulation of cellular excitability and its role in maintaining appropriate activity levels and maturing network excitability.